An apparatus for steering the steerable wheels of a vehicle has been known, which employs a steering wheel. The apparatus converts the rotational motion of steering wheel into the linear motion of a rack shaft situated in a steering gearbox, thus driving a link mechanism coupled with the rack shaft to steer the steerable wheels.
Recently, a steer-by-wire system has been reported, which mechanically separates a steering shaft connected to a steering wheel from a steering mechanism to steer the steerable wheels and instead electrically controls via a control unit a steering motor disposed in a steering apparatus. A steering apparatus of steer-by-wire (hereinafter referred to as a steering apparatus) basically has a steering motor to steer steerable wheels and a sensor for detection of an amount of manipulation of a steering wheel. The steering apparatus controls the steering motor according to the amount detected by the sensor, thereby conveying the manipulation of steering wheel made by a driver to the steerable wheels.
In a steering apparatus of steer-by-wire, since a steering shaft connected to a steering wheel is mechanically separated from steerable wheels, a reaction force acting on the steerable wheels is not conveyed to the steering wheel and thereby a driver experiences difficulty in grasping the conditions of a road surface and the steerable wheels. A steering control apparatus disclosed in Japanese Published Patent Application 10-217998 employs steering reaction force to solve the problem described above. The steering reaction force is generated based on a deviation between an amount of control target corresponding to the steered angle of a steering wheel and an amount of steering representative of an actual amount of displacement of the steering shaft, and in addition based on a speed of change of the deviation. This approach allows a driver to feel a reaction force, which is correlated with a discrepancy between an amount of manipulation of a steering wheel and an actual amount of steering of steerable wheels.
A steer-by wire system, which is able to separate steerable wheels from a controller mechanically, can provide better flexibility for the controller design. A steering apparatus, which uses a joy stick instead of a steering wheel in order to steer the steerable wheels of a vehicle, has also been proposed. Japanese Published Patent Application 9-301193 discloses an example for such a type of steering apparatus.
However, a conventional steering apparatus of steer-by-wire, which generates a reaction force independent of vehicle speed, applies a smaller reaction force to a controller than that applied to a steering wheel during vehicular travel at high speeds, thereby still posing a problem of unstable steering (manipulation of a controller). On the other hand, if a reaction force is adapted to be desirable for a vehicle traveling at high speeds in order to improve the stability of a vehicle, it will pose another problem that heavier feeling in manipulating a controller during vehicular travel at low speeds results in less comfortable steering.
When a force exerted by a road surface on the steerable wheels of a vehicle is conveyed to a controller in the form of reaction force so that a driver can have understanding for the conditions of road surface, it may make him feel uncomfortable if all the forces are conveyed to the controller.